This invention relates generally to ultrasonic liquid level meters of the echo-ranging type, and more particularly to a meter that is compensated for environmental changes in the gaseous medium through which the ultrasonic energy is propogated.
In an ultrasonic liquid level meter of the echo-ranging type, pulse of ultrasonic energy transmitted from a station placed above the liquid and reflected thereby to produce echo pulses which are received at the station. By determining the round trip transmit time of the pulse energy in the gaseous medium above the liquid suface, which transit time depends on the distance between the station and the surface, one is able to provide a reading of liquid level.
The accuracy of an ultrasonic liquid level meter of the echo-ranging type is adversely affected by environmental changes, notably temperature, pressure and chemical composition. These factors alter the velocity of acoustic propogation. For example, the velocity sound in air at 0.degree. C. is 1,087.42 fps, whereas in carbon dioxide it is 1,106 fps. When a meter is installed in an environment in which the chemical nature of the gaseous medium undergoes change, this factor will disturb the level reading unless means are provided to compensate or correct therefor. Similarly, changes in the temperature of the medium or in ambient pressure adversely affects the accuracy of the instrument.
In order to provide a reading in an echo-ranging liquid level meter, which is independent of changes in the propogation medium (air or other gas), the U.S. Pat. No. 3,834,233 to Willis et al discloses a first transducer mounted on top of a tank to direct sound energy down into the tank and to detect an echo from the surface of the liquid therein. To compensate for inaccuracies due to changes in the velocity of the sound, Willis et al positions a second transducer a fixed distance from the first to detect the transmitted wave. Detected signals derived from the two transducers are processed to cancel the effects of any variation in the velocity of sound due to environmental fluctuations.
And in order to obviate the need for a second transducer, the U.S. Pat. No. 4,130,018 to Adams et al places a reflector at the center of the level-detecting ultrasound beam. This reflector is a fixed distance from the transmitting transducer, so that an echo is received from this reflector as well as from the surface of the liquid. To this end, Willis et al mounts his transducer within an elongated tube having at its outlet a reference reflector assembly to reflect back a portion of the radiated energy.
The chief disadvantages of the Adams et al instrument, apart from the additional cost of the tube extension for the transducer, is that the center of the ultrasonic beam is obstructed by the reference reflector. This gives rise to attenuation path distortion of the transmitted ultrasonic energy as well as the received echo energy.